The mechanism (of Delaunay) consists of an articulated rhombus PEQF; on his sides PE and PF (of length m)are fixed two cursors whose distances from P are equal, and that can move in a straight path s. The points P and Q are corresponding in a particular affine tranformation(stetching in direction perpendicular to path). If we attach P at a bar PC, pivoted on C, P moves on a circle and Q moves on an ellipse.